The present invention relates to a dynamic focusing electron gun for color cathode ray tubes (CRTs), and more particularly to a dynamic focusing electron gun which forms high-resolution beam spots throughout a screen.
The resolution of a color CRT depends upon the size and shape of electron beam spots formed on the screen. In order to acquire high-resolution images, the electron beam spots should be as small as possible and their shape should be distorted as little as possible. However, an ordinary color CRT employs a self convergence method in which three electron beams are directed toward a target on the rear surface of the screen through a final accelerating lens of the electron gun, and a deflection yoke which forms a pincushion-typed horizontal deflection magnetic field and a barrel-typed vertical deflection magnetic field as a device for deflecting the electron beams. In this structure, the electron beams directed toward the periphery of the screen are deflected at a relatively large angle, passing through the non-uniform vertical and horizontal deflection magnetic fields. Thus, the electron beams passing the non-uniform magnetic fields are horizontally underfocused and vertically overfocused, and the beam spots formed by the electron beams reaching the screen's periphery are elongated horizontally so that a quite large halo is formed around the spots. There:fore, the portion of the image formed in the periphery of the screen suffers a degree of deterioration in comparison with that formed at the screen's center.
To prevent the peripheral image from deteriorating as described above, a method has been proposed in which a dynamic electric field is used to dynamically control the focusing of the electron beam according to screen areas, so as to form uniform spots throughout the screen. The method is embodied by a so-called dynamic focus electron gun whose various modifications are disclosed in U.S. Pat. Nos. 4,814,670, 4,473,775, 4,771,216 and 4,731,563.
Referring to FIG. 1, an ordinary dynamic focus electron gun comprises a cathode 2, control electrode 3 and screen electrode 4 constituting a triode for forming an electron beam, a static focus electrode 5a, dynamic focus electrode 5b and final accelerating electrode 6 constituting a main lens system for forming a static focus lens and dynamic focus lens.
Control electrode 3 is held at a 0 V potential while a screen voltage of 200-1200 V is applied to screen electrode 4. Static focus voltage Vs and dynamic focus voltage Vd are applied to static focus electrode 5a and dynamic focus electrode 5b, respectively. An accelerating voltage Va of 20-35 Kv is applied to accelerating electrode 6. Generally, dynamic focus voltage Vd is a parabolic waveform in synchronization with a deflection signal applied to the deflection yoke. Its peak voltage is 600 to 800 volts higher than the static focus voltage. Static focus voltage Vs is within the range of 20-35% of accelerating voltage Va.
The waveform of the dynamic focus voltage applied to such a dynamic focus electron gun appears generally as that of FIG. 2. Specifically, static focus voltage Vs applied to static focus electrode 5a is maintained at a predetermined potential. Then, parabolic dynamic focus voltage Vd applied to dynamic focus electrode 5b is varied according to the portions of the screen where the electron beam is to land, and is repeated for every horizontal deflection period (1 H) .
The minimum voltage of each parabolic waveform of one horizontal deflection period may be above, equal to or below the static focus voltage, and is relatively high when the electron beam lands at the extremities of any scanning line compared with when the beam lands at the center thereof. This difference in the minimum voltage is regularly varied by one vertical period (1 V) by frames.
The amplitude I for every horizontal deflection period of the parabolic dynamic voltage is the same throughout the screen, without regard to the landing areas of the electron beam. The maximum and minimum of the dynamic focus voltage is varied by one vertical period. One horizontal scanning line is formed for one horizontal deflection period and a plurality of the horizontal scanning lines are formed for one vertical period, so as to form one frame of image data.
In the graph of FIG. 2, upper and lower trend lines V1 and V2 constituted by the continuum of the parabolic waveform peaks (positive and negative, respectively) show the variation of the peak dynamic focus voltage with respect to the electron beam landing along any vertical line of the screen. (Here, the trend-line peaks occur at the extremities of the vertical lines.) This can be regarded as an assumed virtual vertical dynamic focus voltage. With reference to this, it is noted that the difference of the vertical and horizontal dynamic focus voltages to the static focus voltage is varied in respect to both directions (vertical and horizontal) of the screen. However, the peak-to-peak amplitude of Vd for one horizontal deflection period 1 H in the center of the screen is substantially equal to that in the upper or lower portions thereof. The vertical dynamic focus voltage is applied in a form in which, during the vertical deflection period, the variation rate for the left and right sides of the screen are the same as that for the screen's center. Therefore, the variation rates of upper and lower trend lines V1 and V2, showing the variation of vertical dynamic focus voltage Vd, are equal.
FIGS. 3 and 4 illustrate the waveforms of another conventional dynamic focus voltage.
First, referring to FIG. 3, when the electron beam lands on the center of the screen, the minimum of dynamic focus voltage Vd is lower than static focus voltage Vs, and when the electron beam lands on the upper and lower portions of the screen, the minimum of the dynamic focus voltage is relatively high.
Referring to FIG. 4, when the electron beam lands on the center of the screen, the minimum of dynamic focus voltage Vd is substantially equal to static focus voltage Vs. When the electron beam lands on the upper and lower portions of the screen, the minimum of the dynamic focus voltage is relatively high.
The amplitude of the dynamic focus voltage applied to the conventional dynamic focus electron gun stays constant regardless of the electron beam landing areas of the screen. However, since the distance from the electron beam projecting point (from the electron gun) to the screen varies according to landing position (the screen is aspherical) and since the electron beam is severely distorted due to the deflection yoke, uniform beam spots cannot be obtained throughout the screen. Given the structural limitations of CRTs, the above conventional method for applying voltages cannot realize a good-quality picture.
As shown in FIG. 5, when the focus of the electron beam accommodates the left and right peripheries of a screen 100, that is, at the extremities of a horizontal scanning line 110, the core of the beam spot at the center of scanning line 110 is enlarged, which thus greatly deteriorates the quality of the image formed on the center of the screen. Further, as shown in FIG. 6, when the electron beam is focused along the vertical line passing through the center of the screen, that is, accommodating the center of scanning line 110, the beam spots formed at the extremities of horizontal scanning line 110 exhibit large halos that diminish the quality of the picture.